WFS
http://www.jtsstrength.com/articles/2014/09/04/best-damn-squat-mobility-article-period/
from Mountain Athlete: https://youtu.be/y4lT0UQvJZc
I have had great results with both of the above. For me, it was more about stability and muscle activation that mobility. It is also very common among other athletes I have worked with.
I'm surprised nobody has mentioned Pavel's mobility work..... I don't have direct experience, but it normally gets good reviews.